The invention relates to an ellipsometric apparatus.
An ellipsometric apparatus is disclosed in European patent specification No. 19088. That comparative ellipsometer makes it possible to investigate physical surface properties, for example the thicknesses of layers or films on the surfaces of testpieces or samples, in an eyepiece, directly and without the assistance of expensive electronic evaluation equipment. For that purpose, light which is polarised by a polariser is caused to be reflected at a reference surface with known reflection characteristics, and a surface of a testpiece which is to be investigated, with the angles of incidence at each surface being the same, the reference surface and the testpiece surface being perpendicular to each other with their angles of incidence or the direction of polarisation of the radiation being turned through 90.degree. between the two surfaces in question. If the reflection characteristics of the reference surface and the testpiece surface are the same, for example if a layer or film which is to be found on the surface of the testpiece is of the same thickness as a layer or film on the reference surface, the light is extinguished by an analyser disposed downstream of the testpiece. When using a reference surface having a layer thereon which is of a wedge-like or tapered configuration, together with an associated suitable reading-off scale, the procedure is that, when the thickness of the layer or film on the surface of the testpiece is found to be the same as a given location on the tapered layer on the reference surface, a dark measuring strip can be seen at a given location on the scale, in the eyepiece which is disposed downstream of the analyser. In that way it is possible to measure layer thicknesses of from 2 nm up to 50 .mu.m, with a high degree of accuracy which in many cases may be better than .+-.2%.
In the apparatuses which have been disclosed hitherto in that respect, the degree of measuring accuracy is limited by virtue of the fact that the display of the measured thickness of the layer in question appears in the eyepiece in the form of a dark measuring strip which covers over a certain area on the scale.